This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No. 2001-391194 filed in JAPAN on Dec. 25, 2001, which is herein incorporated by reference.
This invention relates to a liquid epoxy resin composition which has improved adhesion to the surface of silicon chips and especially photosensitive polyimide resins, nitride films and oxide films, and cures into a product having improved resistance to humidity and to thermal shocks at high temperatures above the reflow temperature of 260xc2x0 C., and is thus suitable as a sealant for semiconductor encapsulation. It also relates to a semiconductor device which is sealed with the liquid epoxy resin composition.
The trend toward smaller sizes, lighter weights and increased capabilities in electrical equipment has led to a shift in the dominant semiconductor mounting process from pin insertion to surface mounting. Progress of semiconductor devices toward a higher degree of integration entails the enlargement of dies having a size as large as 10 mm or more per side. For semiconductor devices using such large size dies, greater stresses are applied to the die and the sealant during solder reflow. Such stresses are problematic because separation occurs at the interface between the sealant and the die or substrate, and the package cracks upon substrate mounting.
From the expectation that the use of leaded solders will be banned in the near future, a number of lead-substituted solders have been developed. Since most substitute solders have a higher melting temperature than the leaded solders, it has been considered to carry out reflow at temperatures of 260 to 270xc2x0 C. At such reflow temperatures, more failures are expected with sealants of prior art liquid epoxy resin compositions. Even with flip chip type packages which have raised no substantial problems in the prior art, the reflow at such high temperatures brings about serious problems that cracks can occur during the reflow and the sealant can peel at interfaces with chips or substrates.
An object of the invention is to provide a liquid epoxy resin composition which cures into a cured product that has improved adhesion to the surface of silicon chips and especially photosensitive polyimide resins and nitride films, does not suffer a failure even when the temperature of reflow after moisture absorption elevates from the conventional temperature of nearly 240xc2x0 C. to 260-270xc2x0 C., does not deteriorate under hot humid conditions as encountered in PCT (120xc2x0 C./2.1 atm), and does not peel or crack over several hundred cycles of thermal cycling between xe2x88x9265xc2x0 C. and 150xc2x0 C. Another object of the invention is to provide a semiconductor device which is sealed with the cured product of the liquid epoxy resin composition.
The invention pertains to a liquid epoxy resin composition comprising a liquid epoxy resin, a curing agent, a curing accelerator, and an inorganic filler. It has been found that by using as the liquid epoxy resin a mixture of (a) an epoxy resin containing two or less epoxy functional groups in a molecule and being liquid at normal temperature and (b) an epoxy resin containing two or more epoxy functional groups in a molecule and being solid at normal temperature in a weight ratio of epoxy resin (a)/epoxy resin (b) of from 9/1 to 1/4, having a viscosity of up to 10,000 poises at 25xc2x0 C. as measured by an E type viscometer, while selecting the solid epoxy resin (b) from epoxy resins of the general formulae (1) to (4) shown below and combinations thereof, and compounding the components so that the composition has a toughness K1c of at least 3.0, there is obtained a liquid epoxy resin composition that is effectively adherent to the surface of silicon chips and especially photosensitive polyimide resins and nitride films, does not deteriorate under hot humid conditions as encountered in PCT (120xc2x0 C./2.1 atm), and is fully resistant to thermal shocks. The composition is thus suited as a sealant for large die size semiconductor devices. The epoxy resins (a) and (b) are sometimes referred to as xe2x80x9cliquid epoxy resin (a)xe2x80x9d and xe2x80x9csolid epoxy resin (b),xe2x80x9d respectively. 
Herein R is independently hydrogen, C1-C4 alkyl or phenyl, and x is 0 to 10. 
Herein R1 is hydrogen or a monovalent hydrocarbon group of 1 to 6 carbon atoms, Rxe2x80x2 is hydrogen, methyl or ethyl, and y is an integer of 0 to 6. 
Herein z is an integer of at least 1. 
Herein R1 to R4 are each independently hydrogen or a monovalent hydrocarbon group of 1 to 6 carbon atoms.
Specifically, the epoxy resin (b) containing two or more epoxy functional groups in a molecule and being solid at normal temperature, that is, solid epoxy resin is well known as semiconductor sealant. However, it is not liquid and even when it is mixed with a liquid epoxy resin, the resulting mixture (or liquid epoxy resin composition) has a high viscosity and cures into a brittle product and finds few use as the sealant. Quite unexpectedly, by combining the solid epoxy resin (b) with the liquid epoxy resin (a) in a specific mixing ratio, and preferably by including a specific acid anhydride curing agent so that the cured product may become tough, there is obtained a liquid epoxy resin composition which is more adherent and more resistant to thermal shocks, and maintains excellent properties under hot humid conditions. The composition is thus suited as a sealant for large die size semiconductor devices.
Therefore, the present invention provides a liquid epoxy resin composition comprising a liquid epoxy resin, a curing agent, a curing accelerator, and an inorganic filler as essential components, wherein the liquid epoxy resin comprises a mixture of (a) an epoxy resin containing two or less epoxy functional groups in a molecule and being liquid at normal temperature and (b) an epoxy resin containing two or more epoxy functional groups in a molecule, being solid at normal temperature and selected from epoxy resins of the general formulae (1) to (4) and combinations thereof, in a weight ratio of epoxy resin (a)/epoxy resin (b) of from 9/1 to 1/4; the epoxy resin mixture has a viscosity of up to 10,000 poises at 25xc2x0 C. as measured by an E type viscometer; and the composition has a toughness K1c of at least 3.0.